The present invention relates generally to polyolefin foam/film composite structures and, more particularly, to such composites that are used as flooring underlayments for laminate floors.
Laminate floors are relatively new flooring materials that may be used in place of more traditional materials, such as wood, tile, or vinyl, but are typically constructed to resemble either wood or tile. Laminate floors generally comprise two or more layers, including a top or surface layer and a core layer. The surface layer is a protective, wear-resistant layer which may contain aluminum oxide particles or other materials that form a hard, durable surface. The core layer to which the surface layer is bonded may comprise high density fiberboard. This wood-based material may include a tongue-and-groove design to allow pieces of the flooring to be bonded together with an adhesive. The laminate may also include a bottom layer to help balance the flooring and add strength.
Laminate floors are commercially available from various manufacturers such as Wilsonart International and Mannington, and are designed to be installed as a floating floor, i.e., not nailed or glued to the subfloor. Instead, the flooring is applied over a floor xe2x80x9cunderlayment,xe2x80x9d which is typically a thin layer of polyethylene foam (e.g., less than 0.1 inch), to provide cushioning and sound reduction. When laminate flooring is applied on a concrete sub-floor, it is important that the underlayment also provide a barrier to the passage of water vapor therethrough so that water vapor from the concrete subfloor does not cause the core/fiberboard layer of the laminate floor to deteriorate from rotting. While polyethylene foam provides excellent cushioning and sound reduction, the water vapor transmission rate (WVTR) through polyethylene foam is higher than desired for floor underlayment applications on concrete sub-floors. Further, while many laminate flooring materials include a bottom layer, this layer generally does not provide a barrier to the passage of water vapor.
It has previously been proposed to adhere a sheet of polyethylene film to a sheet of polyethylene foam in order to provide a composite structure having a lower WVTR than a sheet of polyethylene foam alone. While this has proven successful, the WVTR of polyethylene film is such that a thickness of at least 2 mils is required to provide the composite with a sufficiently low WVTR for floor underlayment applications for laminates, which has been determined to be 0.6 grams/100 in2 per 24 hours (measured @ 100xc2x0 F. and 90% relative humidity) or less. It would be desirable to achieve such a low WVTR with a lower film thickness, thereby using less resin to make the film and, as a result, reducing the cost of the floor underlayment composite.
Further, conventional PE foam/PE film underlayments have proven to have less abuse- and tear-resistance than would otherwise be desired, as installation of such underlayments often take place in rather rough building construction environments where the PE foam component of the underlayment can be too easily torn or punctured, thereby compromising the integrity of the water vapor barrier that the PE film is otherwise intended to provide. It has been determined that an increase in the tensile strength and tear initiation resistance of the film component of foam/film underlayment composites would have a beneficial impact on the abuse- and tear-resistance of the underlayment material.
Accordingly, there is a need in the art for a foam/film composite material for laminate floor underlayment use that has a low WVTR (less than about 0.6 g/100 in2 per 24 hours) but which has greater abuse- and tear-resistance (i.e., toughness) and a thinner film component than current PE foam/PE film underlayment materials.
That need is met by the present invention, which provides a composite structure, comprising:
a. a foam sheet comprising polyolefin; and
b. a film sheet adhered to the foam sheet, wherein the film sheet
(1) comprises polypropylene,
(2) is stretch-oriented in at least one direction,
(3) has a thickness of less than 0.9 mil, and
(4) has a larger surface area than the foam sheet such that at least one edge of the film sheet extends beyond a corresponding edge of the foam sheet.
Also provided is a method for making a composite structure, including the steps of providing a foam sheet comprising polyolefin and adhering a film sheet as described above to the foam sheet.
The inventor has found that by using a stretch-oriented, polypropylene film, lower MVTR and greater toughness are achieved in the resultant composite structure than a composite structure formed from a polyethylene film that has not been stretch-oriented, e.g., a polyethylene film formed from a blown tube process that is not xe2x80x9cstretch-orientedxe2x80x9d as that term is defined hereinbelow. As a result, thinner, more economical films can be employed in composite structures for floor underlayment applications while retaining the same and in some cases better properties, e.g., greater toughness, than floor underlayments formed with thicker polyethylene films that are not stretch-oriented.